This invention relates generally to semiconductor manufacture and specifically to a probe card for establishing temporary electrical communication with a substrate, such as a semiconductor wafer. This invention also relates to a method for fabricating the probe card, to a method for testing substrates using the probe card, and to a system for testing substrates that includes the probe card.
During a semiconductor fabrication process semiconductor dice are formed on a wafer. The wafer includes a semiconducting substrate, such as silicon or gallium arsenide, on which integrated circuits are formed. During and following the fabrication process, the wafer must be tested to evaluate the electrical characteristics of the integrated circuits. For example, standard wafer probe is performed to test the gross functionality of each die contained on the wafer. Speed probe is performed to test the speed performance of the dice. Other tests such as full functionality testing, burn-in testing and dynamic burn-in testing are typically performed after the dice have been singulated from the wafer.
Currently semiconductor wafers are probe tested utilizing probe cards. One type of probe card includes needle probes for making temporary electrical connections between contact locations on the dice (e.g., bond pads, fuse pads, test pads) and external test circuitry. The probe card typically includes an insulating substrate, such as a glass filled plastic. The substrate can include electrical traces in electrical communication with the needle probes. In addition, the needle probes can be configured to contact a specific die on the wafer. Typically the wafer or the probe card is stepped so that the dice on the wafer are tested one at a time in sequence. There are also probe cards configured to test multiple dice on the wafer (e.g., 8 or 16).
One problem with this type of conventional probe card is that the planarity and positions of the needle probes can vary. Typically the needle probes must initially be bent by hand into required x and y locations to match the contact locations on the wafer. This is a very labor intensive procedure. Also the z-direction location of the contact locations on the wafer can vary. This can cause inaccuracies in the test results because electrical contact with the different contact locations can vary. Continued use of the needle probes causes deformation and further misalignment of the needle probes. Probe cards with needle probes are thus expensive to fabricate and expensive to maintain.
Another problem with needle probe cards is that the contact locations on the wafer are typically coated with a metal oxide layer. For example, aluminum test pads can be covered with aluminum oxide that forms by oxidation of the underlying metal. This metal oxide is electrically nonconductive and provides a high degree of electrical resistance to the needle probes. In order to ensure accurate test results, the needle probes must penetrate this oxide layer to the underlying metal film. This requires xe2x80x9coverdrivexe2x80x9d and xe2x80x9cscrubxe2x80x9d forces which can damage the contact locations and wafer.
Typically, to penetrate the oxide, the probe card and wafer are brought together until the needle probes contact the contact location. The probe card is then xe2x80x9coverdrivenxe2x80x9d a distance (e.g., 3 mils) which deflects the needle probes and causes them to bend. As the needle probes bend, the ends of the needle probes move horizontally across the contact location causing the ends to scrape over the metal. This causes the ends to break through the native oxide layer and contact the underlying metal. The scrubbing action also displaces some of the metal on the contact location causing a groove and a corresponding ridge.
Still another problem with needle probe cards is that they require the contact locations on the dice to be oversized. In particular due to the inaccuracies in the x-y placement of the needle probes, the contact locations on the dice must made large enough to accommodate alignment variations between the needle probes. This requires that the contact locations be made larger by default, which in turn makes the dice larger.
To overcome some of the problems associated with conventional needle probes, membrane probe cards have been developed. Membrane probe cards are manufactured by Packard Hughes Interconnect of Los Angeles, Calif., and others. Membrane probe cards typically include a membrane formed of a thin and flexible dielectric material such as polyimide. Contact bumps are formed on the membrane in electrical communication with conductive traces, typically formed of copper. The conductive traces electrically connect to external test circuitry.
In general, membrane probes are able to compensate for vertical misalignment between the contact locations on the wafer. In addition, the membrane probe can include a force applying mechanism that allows the contact bumps to penetrate the oxide layer of the die contact locations. Membrane probes usually don""t have the xe2x80x9cscrubbingxe2x80x9d action of needle probes. Rather membrane probes rely on penetration contact bumps to break through the oxide and contact the underlying metal.
One disadvantage of membrane probes is that vertical xe2x80x9coverdrivexe2x80x9d forces are required to penetrate the oxide and make a reliable electrical connection between the contact bumps on the probe and the contact locations on the wafer. These forces can damage the contact locations and the wafer. In addition, the contact bumps and membranes can be repeatedly stressed by the forces. These forces can also cause the membrane to lose its resiliency.
Another disadvantage of membrane probe card is the CTE (coefficient of thermal expansion) mismatch between the probe card and wafer. In particular the copper traces on the probe card will move as a result of temperature fluctuations causing the contact bumps to scrub across the contact locations. This can damage the contact locations or an associated passivation layer. In addition, the masks required to make the membranes are difficult to make with high volume processes. This makes membrane probe cards very expensive.
Yet another disadvantage of probe cards is that dynamic burn-in and full functionality testing are typically performed at the die level rather than at the wafer level. One reason that these test procedures are not performed at the wafer level is that these tests require a large number of connections with the wafer. In addition, a large number of input/output paths between the wafer and test circuitry are required. For example, a wafer can include several hundred dice each having twenty or more bond pads. The total number of bond pads on the wafer can be in the thousands. For some tests procedures an input/output path must be provided to each bond pad. Even with wafer stepping techniques, conventionally formed probe cards, usually do not include enough probes or contact bumps to test groups of dice having a large number of contact locations.
In view of the deficiencies of prior art probe cards, improved probe cards are needed for semiconductor manufacture.
In accordance with the present invention, an improved probe card for testing substrates such as semiconductor wafers, a method for fabricating the probe card, a method for testing using the probe card, and a system for testing that includes the probe card are provided.
The probe card includes an interconnect substrate having patterns of contact members for electrically contacting corresponding patterns of contact locations (e.g., bond pads, fuse pads, test pads) on one or more dice on a wafer under test. The probe card also includes a membrane for physically and electrically connecting the interconnect substrate to a testing apparatus (e.g., wafer probe handler). Preferably the membrane comprises a thin elastomeric tape having metal conductors formed thereon. The membrane can be similar to multi layered TAB tape comprising an elastomeric tape, such as polyimide, laminated with patterns of metal foil conductors.
The probe card can also include a compressible member mounted to a surface (e.g., backside) of the interconnect substrate. The compressible member cushions the forces applied by the testing apparatus to the wafers, and allows the interconnect substrate to self planarize to the contact locations on the wafers. The compressible member can be formed of an elastomeric material, as a gas filled bladder, or as a sealed space for retaining compressed air. The compressible member can also be formed of a metal elastomer to provide heat conduction from the interconnect substrate and wafer.
In an illustrative embodiment of the probe card, the contact members comprise raised members having penetrating projections covered with conductive layers. The penetrating projections can be configured to penetrate the contact locations on the wafer to a self limiting penetration depth. The conductive layers can include a barrier layer, or can be formed of a non-reactive material, to prevent material transfer between the contact members and contact locations during a test procedure. In addition, the contact members can be formed in dense patterns to accommodate testing of dense arrays of dice having dense patterns of contact locations. Furthermore, the contact members can be sized and shaped to minimally damage the contact locations on the wafer yet provide a reliable electrical connection.
Still another aspect of the contact members is that less contact force is required so that multiple dice, up to all of the dice on a semiconductor wafer, can be contacted at the same time. With all of the dice on the wafer contacted at the same time, test signals can be electronically applied and switched as required, to selected dice on the wafer.
In addition to the contact members, the interconnect substrate includes patterns of conductors formed in electrical communication with the conductive layers for the contact members. The patterns of conductors can be configured to establish electrical communication with corresponding patterns of conductive traces on the membrane. The patterns of conductors can also include bonding pads formed on a stepped edge, or grooved portion of the interconnect substrate. The stepped edge or grooved portion provides a recess for bonding of the membrane to the interconnect substrate without interfering with the operation of the contact members. Bonding between the conductors and membrane can be effected by wire bonding, ribbon bonding, microbump bonding or conductive adhesives.
An alternate embodiment probe card includes indentation contact members, configured to establish electrical communication with bumped contact locations (e.g., solder bumps) on a bumped wafer. Another alternate embodiment probe card includes compliant contact members comprising contact pins formed with a spring segment. Another alternate embodiment probe card includes a rigid mounting plate to which the interconnect substrate can be bonded. The mounting plate can also include terminal contacts arranged in a dense array (e.g., pin grid array) for electrically mounting the interconnect substrate to a corresponding socket on the testing apparatus.
The probe card can also include a leveling mechanism for planarizing a location of the contact members with respect to the contact locations on the wafer. The leveling mechanism can include leveling screws that are adjustable to change the planar orientation of the interconnect substrate.
A method for testing a semiconductor wafer in accordance with the invention comprises the steps of: providing a testing apparatus comprising test circuitry and a probe card fixture; providing an interconnect substrate comprising contact members configured to establish temporary electrical communication with contact locations on the wafer; connecting a membrane to the interconnect substrate configured to physically mount the interconnect substrate to the probe card fixture and to provide an electrical path between the contact members and test circuitry; biasing the interconnect substrate against the wafer to form an electrical connection therebetween; and then applying test signals through the membrane and contact members to contact locations on the wafer.
A system for testing a semiconductor wafer in accordance with the invention comprises: a testing apparatus including test circuitry and a probe card fixture; an interconnect substrate flexibly mounted to the probe card fixture, configured to establish temporary electrical communication with contact locations on the wafer; and a membrane for mounting the interconnect substrate to the probe card fixture, and for establishing an electrical connection between the contact members and the test circuitry.